The control of cardiac stimulation implies a regular adjustment of various control parameters. The principal control parameters are the frequency of stimulation and the atrioventricular delay (AVD). These parameters, as well as the decision whether or not to stimulate a ventricle, rely on the control over the presence or absence of a spontaneous ventricular rhythm, associated with an atrial rhythm (that can itself be a spontaneous or stimulated rhythm).
Another control parameter regularly readjusted is the voltage level of the stimulation pulse delivered to the cardiac cavities, namely the ventricular or atrial cavities. This is because the voltage stimulation threshold, also called the “capture threshold”, is a value that can vary with time and the device must deliver a stimulation pulse that exceeds the capture threshold to produce with certainty a depolarization of the myocardiac cavity. It is therefore desirable to be able to reevaluate at regular intervals the stimulation amplitude level by performing a test of the effectiveness of the stimulation threshold, called the “capture test.”
An algorithm for automatically testing the ventricular capture threshold is described, for example, in patent publication WO-A-93/02741 and its corresponding U.S. Pat. No. 5,411,533 (assigned to Ela Médical, the assignee hereof). This algorithm also is used in commercial pacemaker products sold under the Talent™ brand available from Ela Médical, Montrouge, France. This test algorithm uses in particular the detection of the presence or absence of a spontaneous ventricular rhythm associated with a concomitant, spontaneous or stimulated, atrial rhythm.
A clinical follow-up of patients having devices using this algorithm has revealed that, in certain cases, the algorithm for controlling the pacemaker or the capture test algorithm is some times fooled by the occurrence of “fusions”, i.e., stimulation pulses intervening in a concomitant way to a spontaneous ventricular depolarization. Indeed, after a ventricular stimulation, the detected ventricular event (a “QRS complex”) can be either the direct result of this stimulation, taking into account the latency delay existing between these two events, or a spontaneous complex occurring in the same temporal window (i.e., a fusion). The occurrence of a fusion can have a deleterious effect from the point of view of the patient's hemodynamic condition, because of the presence of two contractions very close together of which one is essentially useless.
In the case of a capture test, even if a fusion does not have an adverse hemodynamic effect, it is nevertheless likely to produce an increase of the value of the capture threshold applied relative to the capture threshold actually needed by patient. This will result in a readjustment of the stimulation amplitude to a higher level than is necessary that will be maintained at least for several hours. Although the excessive level is not in itself dangerous, it does increase power consumption over what would be used with a lower stimulation amplitude, and thus reduces the useful lifespan of the implant.